Although prosthetic feet have been in use for many years, not until relatively recently have efforts been made to design the prosthetic feet to dynamically interact with the cyclic loading and unloading of the foot during body movements thereby to more closely simulate natural body movement and gait. To this end, such prostheses typically are configured to store and release energy during normal body movements.
One such type of prosthesis is disclosed by U.S. Pat. No. 4,547,913 which concerns a prosthetic device having an upwardly extending leg portion, a foot portion extending forwardly from the bottom of the leg portion and a heel portion extending rearwardly from the bottom of the leg portion, with all three portions rigidly joined together. The three portions of the prosthesis are composed of elastic, flexible material to absorb strain energy and thereafter release the energy during leg and foot movement. A substantial drawback of the prosthesis disclosed in the '913 patent is that the leg portion of the prosthesis precludes it from serving as only a foot prosthesis. Moreover, the spring rate of the foot portion of the prosthesis appears to vary at a fairly uniform rate along the length thereof. However, to closely simulate normal gait it is desirable that the prosthesis exhibit a relatively low spring rate and thus high compliance during initial footfall followed by a substantially higher spring rate to carry the weight of the amputee without further substantial deflection of the prosthesis to avoid excessive lowering of the hip.
United Kingdom Patent Publication 2,187,102 discloses another type of prosthetic foot having a keel and an underlying leaf spring stiffener located beneath the keel. The keel includes front and rear snubbers at the extremities thereof. During normal ambulation the leaf spring is said to provide the primary path by which ground reaction is transmitted to the keel but at high levels of ground reaction the ends of the leaf spring flex upwardly to contact against the overhead snubbers whereby the ground reaction is transmitted directly to the keel. This construction is said to provide substantial compliance during normal movement, such as walking, while also accommodating the higher loads generated during jumping or similar movements. However, a serious drawback of this type of prosthesis is that when the underlying leaf spring deflects upwardly to bear against the keel snubber, the resistance to further deflection abruptly increases significantly. Moreover, the prosthetic foot is composed of numerous separate components which must be individually manufactured and then fastened together by bolts that may loosen over time.
A further type of prosthetic foot is disclosed in U.S. Pat. No. 4,645,509, which has been assigned to Model And Instrument Development Corporation, of Seattle, Washington, which is also the assignee of the present invention. In the '509 patent, the prosthesis includes a monolithic cantilever keel having an attachment flange for connection to an upper prosthesis, an arched heel portion curving initially downwardly and rearwardly from the attachment flange and then forwardly to join a forefoot portion. The curved heel and the forefoot portion are shaped and dimensioned to have a substantially uniform bending stress distribution and strain energy storage throughout their lengths. A significant drawback of the keel of the '509 patent is that the attachment flange thereof is at a relatively high elevation which is substantially above the location of the ankle joint in a natural foot. Moreover, primarily due to relatively large size of the keel, the prosthetic foot of the '509 patent is relatively heavy, making it tiresome to wear over extended periods or during strenuous activities. Since the prosthetic foot is continually accelerated and decelerated during body movement, it is imperative that the foot be as light in weight as possible.